1. Field of the Invention
The present invention relates to fixed optical attenuators used in communication systems using optical fibers. In particular, the present invention relates to an optical attenuator including an attenuating fiber provided with an optical attenuation function.
2. Description of the Related Art
Various types of attenuating fiber assembly have been proposed. The attenuating fiber assemblies are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-61632 and Japanese Patent Application No. 2000-194443 by the same applicant of the present invention, in which a desired attenuation can be obtained at high accuracy.
In order to describe problems which are solved by the present invention, a known typical optical attenuator is briefly described with reference to FIG. 6 which shows a ferrule assembly including an attenuating fiber. The ferrule assembly including an attenuating fiber shown in FIG. 6 can be used in a fixed optical attenuator for FC-type, SC-type, MU-type, or LC(trademark)-type connector. A ferrule 1 shown in FIG. 6 is provided with a central hole 2 for receiving an attenuating fiber 3. The attenuating fiber 3 is inserted into the central hole 2 and is affixed by an adhesive. The ferrule 1 is mirror-finished at end faces 4 and 5 thereof by polishing after the attenuating fiber 3 is fixed to the ferrule 1.
The ferrule 1 fixed to an optical fiber is inserted into a through-hole 7 of a holding sleeve 6 either before or after the ferrule 1 is polished. The shape of the holding sleeve 6 may be modified according to the use of the optical attenuator. A flange 6a shown in FIG. 6 may be omitted.
The known optical attenuator which is used for various types of connector, as described above, has a problem regarding the length of the ferrule. Ordinary ferrules which are presently widely used each have an outer diameter d of 2.5 mm and a length Lc of 10.5 mm. However, a significantly longer ferrule is used in a fixed optical attenuator for the FC-type, SC-type, or MU-type optical connector. For example, in the fixed optical attenuators for the FC-type and SC-type connector, the ferrules each having a length of 23 mm are used by reason of the configuration. Generally, the ferrules for the MU-type connector each have the outer diameter d of 1.25 mm and the length Lc of 7 mm. However, the length of the ferrule used in an optical attenuator for the MU-type connector must be significantly larger, such as a length L of approximately 15 mm.
Such ferrules are manufactured generally with a zirconia ceramic either by extrusion or by molding. Even by other method, the length of the ferrules for the optical attenuators is more than twice as long as the length of the ferrules for the ordinal connectors. Therefore, it is difficult to manufacture the ferrules for the optical attenuators each in highly accurate sizes and in a stable manner in large quantities. As a result, there is a risk in that the supply quantity of the ferrules is limited, thereby increasing manufacturing costs.
Accordingly, it is an object of the present invention to provide an optical attenuator arranged so that the problems found in the known optical attenuator described above can be solved.
It is another object of the present invention to provide an optical attenuator enabled to be manufactured at low costs by using ferrules having relatively short axes which can be manufactured in a stable manner in large quantities.
In order to achieve the above objects, an optical attenuator according to the present invention includes a holding sleeve; a pair of ferrules inserted into and fixed to the holding sleeve such that the ferrules oppose each other at base ends thereof at a predetermined distance between each base end; and an attenuating fiber inserted into and fixed to the pair of ferrules such that ends of the attenuating fiber are exposed.
In the optical attenuator according to the present invention, a soft resin is provided in a space between the base ends of the ferrules.
In the optical attenuator according to the present invention, the soft resin comprises an elastic material including rubber or gel.
In the optical attenuator according to the present invention, the soft resin comprises a filler containing a light-absorptive material.
To these ends, an overall length L of the optical attenuator according to the present invention is set so as to satisfy an expression L greater than L1+L2 in which L1 denotes a length of one ferrule and L2 denotes a length of another ferrule, the ferrules being included in the optical attenuator. The value Lxe2x88x92(L1+L2) may vary, as needed.